The exogenous Moloney leukemia virus (=M-MuLV) will be used to address three general problems of gene activation and gene function in mammalian development: (1) as a model gene to analyze molecular parameters of gene activation in mouse development; (2) as an insertional mutagen to induce developmental mutations; (3) as a vector to correct genetic defects in animals. (1) A collection of 14 substrains of mice (termed Mov-1 to Mov-14), each carrying a single M-MuLV provirus at a distinct Mendelian locus, will be used for analyzing the tissue specific expression, methylation and chromatin structure of both the proviral genome and the host DNA flanking the provirus. The aim of these experiments is to understand the influence of the chromosomal position on developmental expression of a gene which is experimentally inserted into the germ line. The provirus in Mov-13 mice has integrated into the Alpha1 (I) collagen gene blocking its transcription and resulting in a recessive lethal mutation. To understand the role of collagen in embryogenesis, organ cultures will be established in vitro from embryos prior to developmental arrest. The proviral genomes carried on the X-Chromosome of Mov-14 mice will be used as a non-selective marker to study the process of X-inactivation. (2) Retroviruses will be microinjected into mid-gestation mouse embryos to obtain additional embryonic mutations in an analogous approach as that used to derive the Mov-13 substrain. (3) The Mov-13 substrain will be used for exploring the feasibility of correcting a defined genetic defect by introducing the wild type collagen gene into animals. This will be attempted by microinjecting a retrovirus vector which transduces the collagen gene into Mov-13 embryos prior to developmental arrest.